The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are illustrated in FIG. 1. They are seed layer 11 on which is antiferromagnetic layer 12 whose purpose is to act as a pinning agent for a magnetically pinned layer. The latter is a synthetic antiferromagnet formed by sandwiching antiferromagnetic coupling layer 14 between two antiparallel ferromagnetic layers 13 (AP2) and 15 (AP1).
Next is a copper spacer layer 16 on which is low coercivity (free) ferromagnetic layer 17. A contacting layer such as lead 18 lies atop free layer 17. When free layer 17 is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Earlier GMR devices were designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also emerged. CPP GMR heads are considered to be promising candidates for the over 100 Gb/in2 recording density domain (see references 1-3 below).
A routine search of the prior art was performed with the following references of interest being found:
No references were found that disclosed a specific percentage of Fe in the free layer. In U.S. Pat. No. 6,680,831, Hiramoto et al. disclose a simplified SV structure with only a pinned layer and a free layer separated by an intermediate, non-magnetic, layer. The pinned layer could be FeCo containing at least 50% Fe or Co. In U.S. Pat. No. 6,529,353, Shimazawa et al. and in U.S. Pat. No. 6,519,124, Redon et al. teach that the free layer may be a laminate of FeCo and NiFe. U.S. Patent Application 2002/0048127, Fukuzawa et al. teach a CoFeNi free layer for a higher rate of change in MR than CoFe/NiFe. Both Redon and Shimazawa disclose a laminated CoFe/NiFe free layer. Unless otherwise specified, CoFe usually means Co90Fe10; CoFe/NiFe composited free layers of this type are well known for spin valve applications.
An improved free layer in a CPP spin valve needs to achieve three objectives:    1) higher CPP GMR ratio;    2) low coercivity i.e., good magnetic softness; and    3) low positive magnetostriction.None of the prior art inventions listed above achieve all three of these, particularly the low positive magnetostriction